The invention relates generally to acoustic resonators and more particularly to approaches for supporting an acoustic resonator on a substrate.
Acoustic resonators that are formed of thin films may be used in a number of applications that require a precisely controlled frequency. A Thin Film Bulk Acoustic Resonator (FBAR) or a Stacked Thin Film Bulk Acoustic Resonator (SBAR) may be used as a filter in a cellular telephone or other device in which size, cost and frequency stability are important factors.
An FBAR includes a thin film of piezoelectric material between two conductive electrodes, while an SBAR includes additional layers of piezoelectric material, with each such layer separating two electrodes. The active layers of an FBAR or SBAR are suspended in air by supporting the layers around the perimeter. The air/resonator interfaces at both sides of the stack of layers trap the energy that is generated during operation.
When a time-varying electrical field is created by applying a signal across two electrodes that are separated by a piezoelectric layer, the piezoelectric material converts some of the electrical energy into mechanical energy in the form of sound waves. The sound waves propagate in the same direction as the electrical field and are reflected at the air/resonator interfaces. For a properly fabricated FBAR or SBAR, the sound waves will have a particular mechanical resonance.
An FBAR or SBAR can be used as a filter, since it will function as an electronic resonator when allowed to operate at its mechanical resonant frequency. At this mechanical resonant frequency, the half wavelength of the sound waves propagating through the resonator is approximately equal to the total thickness of the resonator for a given phase velocity of sound in the piezoelectric material. Since the velocity of sound is many orders of magnitude smaller than the velocity of light, the resulting resonator can be compact. A resonator for applications in which a frequency in the gigahertz range is desired may be constructed with physical dimensions less than 100 microns in diameter and a few microns in thickness.
An FBAR is conventionally fabricated on the surface of the substrate by depositing the bottom electrode, the piezoelectric film, and then the top electrode. Therefore, a top air/resonator interface exists and only the bottom interface requires some design selections. There are several known approaches for obtaining the desired characteristics at the bottom interface.
The first approach involves etching away the substrate material from the back side of the substrate. If the substrate is silicon, the silicon is etched away from the back side using a KOH etch, which is a strong base etchant. This approach is described in detail in U.S. Pat. No. 5,587,620 to Ruby et al. With reference to FIG. 1, a layer 10 of Si3N4 may be deposited on a top surface of a silicon substrate 12. The back side of the substrate 12 is then etched using the KOH. Preferably, approximately 80% of the silicon substrate is removed during the etching step, leaving a remainder 14 that provides structural stability. The metallization of the bottom electrode 16 is then formed on the Si3N4 layer 10. Aluminum nitride (AlN) may then be deposited as the piezoelectric layer 18. A second electrode 20 is subsequently formed on the surface of the piezoelectric layer 18. As shown in FIG. 2, if an SBAR is to be fabricated, rather than an FBAR, a second piezoelectric layer 22 and a third electrode 24 are also formed.
After completing the fabrication of the third electrode 24, the remainder 14 of the silicon within the previously etched cavity is removed by a slow etching process that is more easily controlled than the KOH etch. For example, a tetra-methyl-ammonium hydroxide (TMAH) etching solution may be used, since it is less likely to attack the AlN of the piezoelectric layers 18 and 22. The result is that an SBAR 26 of FIG. 2 is formed.
One concern with this first approach is that it results in a relatively low fabrication yield. The cavities that are formed through the wafer 12 render the wafer very delicate and highly susceptible to breakage. Furthermore, the wet etching using KOH forms side walls having 54.7xc2x0 slopes. This limits the ultimate density of fabricating the acoustic resonators on a given sized wafer. For example, devices with lateral dimensions of approximately 150 xcexcmxc3x97150 xcexcm that are built on a standard 530 xcexcm thick silicon wafer require a back side etch hole that is roughly 450 xcexcmxc3x97450 xcexcm. Consequently, only approximately 11% of the wafer can be productively utilized.
A second approach to forming the air/resonator interfaces is to create an air bridge type FBAR/SBAR device. Typically, a sacrificial layer is deposited and the acoustic resonator layer stack is then fabricated on top of the sacrificial layer. At or near the end of the process, the sacrificial layer is removed. Since all of the processing is completed on the front side, this approach does not suffer from having two-sided alignment and large area back side cavities. However, this approach has other inherent difficulties. First, the method is difficult to practice on large devices. Typically, the sacrificial layer is thermally grown silicon dioxide that is subsequently removed using hydrofluoric (HF) gas, which has an etch rate in the order of 1000 to 3000 xc3x85/minute. To etch beneath device areas that are in the order of 150 xcexcmxc3x97150 xcexcm or larger, an etch time greater than 500 minutes is required. In addition to being excessively long, the exposure of the metal electrodes to the etchant for periods in excess of 30 minutes leads to the delamination of the metal electrodes from the piezoelectric material.
A third approach is sometimes referred to as the xe2x80x9csolidly mounted resonatorxe2x80x9d (SMR), since there are no air gaps at the bottom of the device. A large acoustic impedance at the bottom of the device is created by using an acoustic Bragg reflector. The Bragg reflector is made of layers of alternating high and low acoustic impedance materials. Each thickness is fixed to be at the quarter wavelength of the resonant frequency. With sufficient layers, the effective impedance of the piezoelectric/electrode interface is much higher than the device acoustic impedance, thereby trapping the sound waves effectively within the piezoelectric layer. While this approach avoids some of the problems discussed with regard to creating a free-standing membrane, it includes difficulties. The choice of materials used in the Bragg reflector is limited, since metal layers would form parasitic capacitors that would degrade the electrical performance of the device. Moreover, the degree of difference in the acoustic impedance of layers made by the available dielectric materials is not large, so that more layers are needed. This complicates the fabrication process as the stress on each layer must be well controlled. After many layers, the device is not conducive to integration with other active elements, since making vias through a large number of holes is difficult. Furthermore, devices of this type are reported to have significantly lower effective coupling coefficients than devices having air bridges.
Acoustic resonators may be used alone or in combination. For example, a bandpass filter is formed by electrically connecting several resonators to provide a desired filter response. Several filter topologies are possible. One favored topology is the half-ladder topology, where a group of resonators are connected in series (series resonators) and in between the series resonators are shunt resonators that are connected to ground. The series resonators are fabricated such that their resonant frequency is approximately 2% to 3% higher than the shunt resonators. Since the thickness of the piezoelectric layer can be the same for the series and shunt resonators, the piezoelectric deposition is often xe2x80x9csharedxe2x80x9d between resonators. In fact, it is tempting for a designer to use the piezoelectric layer as a membrane that provides the chief supporting structure spanning the backside KOH-etched cavities that were described above. This xe2x80x9csharingxe2x80x9d of the piezoelectric layer helps to mitigate the loss of board real estate that results from the 54.7xc2x0 etch angle.
Regardless of the approach for forming the air/resonator interface, the conventional thinking is that the top and bottom electrodes should be on the opposite sides of only free-standing regions of the piezoelectric layer. This isolates the area in which acoustic energy is being generated to free-standing regions, so that the acoustic energy is less likely to be lost into the underlying substrate. It is also conventional thinking that as a result of the goal of achieving component minimization, providing separate resonators with separate cavities would be undesirable.
What is needed is a method of fabricating an acoustic resonator having a free-standing portion that is able to withstand a wide range of stresses, both compressive and tensile, and which exhibits a relatively high figure of merit (Q).
A filter is formed using robust and high Q acoustic resonators, where each resonator has its own cavity and includes a bottom electrode which extends across substantially the entirety of an open area that is formed within a supporting substrate. The bottom electrode contacts all sides of the open area, so that it includes an unsupported interior region within supported peripheral regions. The bottom electrode is one layer of a layer stack that includes at least one piezoelectric layer and at least one other electrode layer. This results in a significant improvement relative to acoustic resonators in which the bottom electrode has at least one edge within the unsupported region of the layer stack.
In one embodiment of the invention, the open area is formed within the substrate by etching a cavity from the surface on which the acoustic resonator layer stack is to be deposited. The cavity extends only partially through the substrate, which may be a silicon wafer. The cavity is then filled with a sacrificial material which is subsequently removed.
The bottom electrode is preferably formed directly atop the sacrificial material, but an intermediate layer may be included. The bottom electrode extends beyond the sacrificial material at all sides of the cavity. If the cavity is formed by an anisotropic etching step, the cavity will likely have at least three sides. Thus, the bottom electrode will have a supported interior region and three or more substrate-supported peripheral regions.
The piezoelectric layer is formed atop the bottom electrode. Preferably, the piezoelectric layer will extend beyond all edges of the bottom electrode. The piezoelectric material (e.g., AlN) is a columnar material that readily steps down at the edges of the bottom electrode. This characteristic of the piezoelectric material causes a reduction in performance when an edge of the bottom electrode terminates within the cavity region, as in the conventional approach. Using this conventional approach of having an unsupported step down, the distorted and poorly formed columnar piezoelectric membrane at the unsupported step edge will adversely affect electrical performance. Furthermore, the step down creates a mechanical weakness in the structural membrane, thereby potentially leading to cracks. On the other hand, by providing overlap of the bottom electrode onto all of the sides of the cavity, the figure of merit (Q) is increased.
Preferably, the top electrode includes sides that reside within the area defined by the cavity. For example, three sides of the top electrode may be within the xe2x80x9cframexe2x80x9d of the cavity, so that only one edge overlaps the edge of the cavity and resides on the portion of the silicon that is in contact with the layer stack. This reduces the effects of any parasitic FBARs or SBARs that are formed at the bottom electrode step edges.
After the layer stack has been deposited, the sacrificial material is removed from the cavity. Release holes for removing the material may be formed within the layer stack. However, the preferred embodiment limits the release holes to the perimeter of the cavity. It has been discovered that this enhances the Q of the acoustic resonator, since there are fewer internal discontinuities that will cause conversion or loss of lateral mode energy.
In one embodiment, the bottom electrode includes a serpentine edge that leaves a portion of one side of the cavity free of overlap by the bottom electrode, so that a top electrode may overlap this portion. Thus, the top and bottom electrodes can overlap the same side without sandwiching the piezoelectric layer outside of the unsupported interior region.
One advantage of the invention is that by placing the step edge of the bottom electrode and piezoelectric material away from the cavity, the free-standing portion of the acoustic resonator becomes much stronger and is able to withstand a wider range of stresses, both compressive and tensile. Another advantage is that moving the bottom electrode step edge away from the cavity increases the Q. In the conventional approach of placing the edge of the bottom electrode within the cavity, any lateral mode energy that is generated will be reflected from the poorly formed piezoelectric material at the step edges. Some of the energy is mode converted and lost, while another portion is directly converted to heat by incoherent collisions with voids and impurities found at the poorly formed edge. However, by placing the edge onto the surface of the substrate, the edges of the cavity function as highly reflecting edges that reflect the energy back into the free-standing portion of the acoustic resonator. This reflection returns some of the energy from the parasitic lateral mode.
The range of film stresses tolerated by an acoustic resonator formed in accordance with the invention was considerable. Devices were formed with film stresses as high as 6*109 Dynes/cm2 compression and nearly 1010 Dynes/cm2 tensile. Other designs began to fail (i.e., crack or buckle) at much lower film stresses. Improvements in Q were at least a factor of two. It was found that a further 10% to 20% improvement was a result of moving the release holes from the center of the free-standing portion to the edges of the cavity.